Euphausiid shrimps bear light organs, two pairs in the thorax, and four similar single organs in the abdomen: A pair on the eyestalks differ in structure from these others, and are not considered further by us. In most species the size of the organs is related to that of the animal, but they are relatively smaller in deeper living species. In some species certain light organs are larger in males. The flashes of light emitted begin abruptly, remain steady for several seconds and die away slowly. Euphausiids flash in response to light, including that of another euphausiid, and to certain drugs, notably 5 hydroxytryptamine and certain analogues.
The photophores contain a lantern where the light is generated, and three separate optical components which reflect and refract the light out of one pole of the organ. Our interpretation of the structures suggests that light generation is probably controlled by alteration of the blood flow through the lantern, and that this in turn is under nervous control.